The present invention relates generally to the field of auditory measurement. More particularly, the present invention pertains to auditory measurement systems and methods, such as those that may be useful in performing assessments of a patient's auditory function including the fitting of a hearing aid, the screening of a patient's hearing, the diagnosis of middle-ear dysfunction, etc.
The peripheral auditory system includes the external ear, middle-ear, cochlea, the auditory nerve and specific regions of the brain serving auditory perception. The external ear consists of the pinna, which projects from the side of the head and functions as a collector of sound energy, and the ear canal, which leads from the bottom region of the pinna and transmits sound to the eardrum (or tympanic membrane). Functional assessment of the peripheral auditory system is important to identifying and rehabilitating hearing pathologies. A device to screen and diagnose middle-ear dysfunction is valuable in the identification of a hearing disorder and in the clinical management of an existing hearing disorder.
A standard clinical test of middle-ear functioning is an acoustic immittance, test, which refers to one or both of an acoustic admittance or an acoustic impedance test performed in the ear canal of a patient (ANSI S3.39-1987. American National Standard Specifications for Instruments to Measure Aural Acoustic Impedance and Admittance (Aural Acoustic Immittance). An acoustic immittance test includes tympanometry, which is the measurement of acoustic immittance in the ear canal as a function of air pressure, herein termed static pressure, as the static pressure is varied within the ear canal (ANSI S3.39-1987). The acoustic impedance, when expressed as a complex number with two components, is the exact inverse of the acoustic admittance. Thus, knowledge of the acoustic admittance constitutes knowledge of acoustic impedance. In many respects herein, the terms acoustic immittance and acoustic admittance can be used interchangeably. It is understood that the term static pressure refers to the total pressure of air in the ear canal that is slowly varying with respect to the acoustic variations of the stimuli used in acoustic immittance testing. A common stimulus used in clinical tympanometry is a 226-Hz tone, which has a period of approximately 4.4 ms. As long as the total pressure of air is varying slowly with respect to such a 4.4 ms duration, it is common to refer to this total pressure as a static pressure of air. To those skilled in the art, a tympanometry test is a measurement of acoustic admittance or impedance.
In a clinical (immittance) tympanometry test, a probe is inserted into the ear canal in a substantially leak-free manner, and a static pressure pump adjusts the static pressure just above and below the ambient, atmospheric pressure. This creates a static pressure difference across the eardrum. While the pump pressure is varied, a low-frequency sinusoidal sound is presented, typically at a low frequency of 226 Hz. As the static pressure exerts forces on the eardrum, an acoustic response to the low-frequency tone (e.g., measured by a microphone in the probe) varies in the normal-functioning ear. A tympanogram, which is a result of the tympanometry test, is often represented by a plot of the calibrated microphone response to the tone as a function of static pressure
This calibrated microphone response is called an acoustic admittance, which is an acoustic transfer function of the ear. The acoustic admittance is defined as the ratio of the acoustic volume per unit time swept out in response to an acoustic pressure signal. The aural acoustic admittance, which is the acoustic admittance measured in the ear canal at one specific location, typically at the tip of the probe, has the property that it is highly sensitive to its choice of measurement location in the ear canal. Clinical immittance tympanometry is usually performed at a frequency of 226 Hz (e.g., see ANSI S3.39-1987), for which a simple compensation mechanism can be used, at least in the ears of older children and adults, to transform the aural immittance response from the probe tip to the eardrum. The eardrum is the port into which acoustically transmitted signals are input from the ear canal into the middle ear, and thus the eardrum location is preferred for a test of air-conducted middle-ear functioning. At frequencies an octave or so higher than 226 Hz, the compensation procedure recommended in ANSI S3.39-1987 becomes increasingly inaccurate. While various forms of multifrequency tympanometry exist, in which acoustic immittance is measured at frequencies of 660 Hz and higher frequencies up to approximately 2000 Hz, a practical difficulty in multifrequency tympanometry is the interpretation of the response at the probe tip in terms of the response at the eardrum, which is the preferred site at which to assess middle-ear functioning. One approach is to calculate a middle-ear resonance frequency from a multifrequency tympanogram, which can indicate some forms of middle-ear dysfunction, and another approach is to classify immittance tympanograms in terms of their shape. The latter classification scheme has interpretative problems at higher frequencies in the range important for speech perception and in responses measured in infants and young children.
A standard aural acoustic immittance test may include immittance tympanometry and an acoustic immittance test of the middle-ear muscle reflex (MEMR). This MEMR is a change in the tonus of a middle-ear (stapedius) muscle in response to a stimulus, often called the activator signal, which is the elicitor of the MEMR. The muscle contractions associated with activation of the MEMR are tested based on a change in the acoustic immittance, and such acoustic reflex testing is part of the acoustic immittance test battery (ANSI S3.39-1987). Thus, to one skilled in the art of audiology, a tympanometry test is an acoustic immittance test at varying pressure and an acoustic reflex test is based on a change in the acoustic immittance produced by presentation of a MEMR activator signal (pure tones and broadband noise are commonly used activators).
Because acoustic immittance testing is commonly used only at low frequencies, the associated acoustic-reflex test in the acoustic immittance test battery is also performed at low frequencies, commonly using the acoustic admittance change at 226 Hz. Such an acoustic reflex test using acoustic admittance change has the disadvantage that the change in response at 226 Hz is typically smaller than the change at a higher probe frequency. As such, acoustic-reflex thresholds measured at this frequency may be higher than acoustic-reflex thresholds measured at higher frequencies.
More generally, an acoustic transfer function of the ear is any ratio of acoustical variables, with each acoustical variable measured in the ear canal. Two examples of acoustic transfer functions have been described, the acoustic admittance and acoustic impedance. Another important acoustic transfer function of the ear is acoustic reflectance, which is the ratio of the reflected to the incident pressure signal that propagates in the ear canal between the probe and eardrum. Each reflected and incident signal is measured at the same location in the ear canal so that the pressure reflectance is an acoustic transfer function at a particular location in the ear canal.
Like acoustic immittance, the acoustic reflectance can be expressed at each frequency by a complex number, which has real and imaginary components, or, alternatively, magnitude and phase components. Unlike any of the components of acoustic immittance, the acoustic reflectance has the property that its magnitude is approximately independent of the measurement location in the ear canal (Stinson M R, Shaw E A G, and Lawton B W (1982), “Estimation of acoustical energy reflectance at the eardrum from measurements of pressure distribution in the human ear canal”, J. Acoust. Soc. Am. 72, 766-773), as long as the ear-canal walls are sufficiently rigid, as is the case for older children and adults but not necessarily for infants (Keefe D H, Bulen J C, Arehart K H, and Burns E M (1993), “Ear-canal impedance and reflection coefficient in human infants and adults”, J. Acoust. Soc. Am. 94, 2617-2638).
The energy reflectance, which is also termed power reflectance, is the squared magnitude of the pressure reflectance, and is thus also insensitive to ear-canal measurement location. This means that a measurement of energy reflectance at the probe tip is approximately equal to the energy reflectance at the eardrum, and is thus a direct measurement of the acoustic functioning of the middle ear at the ear drum. As such, the energy reflectance of the middle ear can be interpreted at high frequencies, without the need for a compensation procedure to transform the response between the probe tip and eardrum.
The energy reflectance varies between zero and one in the absence of internal sources of energy within the cochlea, such that an energy reflectance of zero means that no energy is reflected from the eardrum and an energy reflectance of one means that all the energy is reflected from the eardrum. It follows that energy reflectance is advantageous for assessing middle-ear functioning over the wideband range of speech frequencies up to 4000 or 8000 Hz, or even higher depending on the measurement device, whereas acoustic immittance at the probe tip is limited to lower frequencies.
A wideband acoustic admittance can also be calculated up to 4000 or 8000 Hz, or even higher frequencies, using the similar measurement principles underlying wideband acoustic reflectance (Keefe D H, Ling R, and Bulen J C (1992), “Method to measure acoustic impedance and reflection coefficient”, J. Acoust. Soc. Am. 91, 470-485). By measuring the distance between the probe tip and eardrum and using an interpretative model of acoustic transmission in the ear canal, the wideband acoustic admittance response at the probe tip can be transformed to a wideband acoustic admittance response at the eardrum. Such a wideband analysis is distinct from any compensation procedure for acoustic immittance known to those skilled in the art of acoustic immittance, and thus the term wideband acoustic admittance is fundamentally different from aural acoustic admittance used in aural immittance test devices.
Based on a measurement of wideband acoustic reflectance, the complex components of wideband acoustic admittance can thus be calculated, for which the real component is called the acoustic conductance. The product of acoustic conductance and the squared root-mean squared magnitude of the acoustic pressure is the power absorbed by the ear (Keefe et al., 1993). Because the power dissipated in the ear canal and at its walls is small over a wide frequency range, conservation of energy implies that the power absorbed by the ear at the probe tip is equal to the power absorbed by the ear at the eardrum. Thus, this absorbed power, like energy reflectance, is a second quantity that is insensitive to the measurement location. The acoustic intensity is the rate at which acoustic energy flows per unit through a cross-sectional area. Because the acoustic energy flow in the ear canal is predominantly of the plane-wave form, it follows that the acoustic intensity is equal to the ratio of the absorbed power and the cross-section ear-canal area at the location at which the absorbed power is measured.
With the approximation that no power is dissipated in the ear canal and at its walls, conservation of energy also requires that the incident energy to the ear canal at any location is equal to the sum of the reflected and transmitted energies. It follows that the energy transmittance, which is defined as the ratio of the transmitted energy traveling in the ear canal towards the eardrum to the incident range, is equal to one minus the energy reflectance (Keefe, D. H. and Simmons, J. L. (2003), “Energy transmittance predicts conductive hearing loss in older children and adults,” J. Acoust. Soc. Am. 114, 3217-3238). Because the energy reflectance is approximately equal at the probe tip (or other measurement location) and eardrum, the energy transmittance is also equal at the probe tip and eardrum. This energy transmittance can also be termed the energy absorptance, because the eardrum absorbs all the transmitted energy (aside from small correction terms due to otoacoustic-emission signals evoked or spontaneously present in the cochlea). This energy transmittance is also called power absorption.
Various patents describe systems and methods for measuring an acoustic reflectance of the ear (e.g., as a function of static pressure and stimulus parameters including time, frequency or stimulus level). For example, U.S. Pat. No. 5,594,174 to Keefe issued 14 Jan. 1997, entitled “System and Method for Measuring Acoustic Reflectance;” U.S. Pat. No. 5,651,371 to Keefe issued 29 Jul. 1997, entitled “System and Method for Measuring Acoustic Reflectance;” and U.S. Pat. No. 5,792,072 to Keefe issued 11 Aug. 1998, entitled “System and Method for Measuring Acoustic Reflectance,” all describe various acoustic reflectance measurement systems and methods.
An important step in any device to perform an acoustic reflectance test is a preliminary calibration, which allows the device to measure the acoustic reflectance data in a test ear in terms of the measured microphone pressure at the probe tip and calibration parameters, which are output from the calibration. The calibration of the acoustic reflectance device is typically performed at one stimulus level, and the acoustic reflectance test in the ear is also performed at one stimulus level. The calibration and ear-testing stimulus levels are the same, at least in one preferred embodiment.
In an alternate embodiment, the stimulus levels differ. The difference in stimulus levels is applied as an additional variable to the calibration parameters by the calibration step. For example, it may be convenient to perform the reflectance test in an infant's ear at a higher stimulus level, because the internal physiologic noise level (produced by the cardiovascular and respiratory systems) is higher in infants than in adults. Three possible choices of calibration parameters are the source incident pressure and source reflectance, the Norton source volume velocity and Norton source admittance, and the Thevenin source pressure and Thevenin source impedance. Other choices of calibration parameters may be equivalent to any of these three sets of calibration parameters, which are each equivalent to one another. A higher stimulus level in testing the infant's ear is achieved by increasing the source incident pressure, the Norton source volume velocity or the Thevenin source pressure, respectively.
It should be understood that, in view of the relationships described above for single-frequency and wideband acoustic transfer functions, a device to measure acoustic reflectance is capable of measuring a wideband response of any or all of acoustic reflectance, energy reflectance, energy transmittance, acoustic admittance, acoustic impedance, absorbed power, and acoustic intensity. Thus, an acoustic reflectance test implies the ability to output the acoustic reflectance and any or all of these other acoustic transfer functions and related responses.
The use of the term “tympanometry” in this application has a different and more general meaning than the use of the term by one skilled in the art of audiological measurements, to whom tympanometry means a pressurized acoustic immittance measurement. Tympanometry in this application refers to the measurement of an acoustic transfer function or sound pressure as a function of frequency or time, and as a function of static pressure, which is varied within the ear canal at a rate much more slowly than the longest period of the acoustic stimulus, which is the period of the lowest frequency for which significant energy is present in the stimulus spectrum. Preferably, the lower limit of the frequency range of tympanometry measurements is approximately 200 Hz, so that this longest period does not exceed 5 ms. The restriction of tympanometry measurements to frequencies at and above 200 Hz is reasonable because middle-ear functioning is slowly varying at low frequencies and because the internal noise at the output of the microphone, which has contributions from measurement system noise, environmental noise, and physiologic noise sources within the patient, becomes large at frequencies below 200 Hz. A reflectance tympanometry test was first described by Keefe D H and Levi E (1996) (“Maturation of the middle and external ears: Acoustic power-based responses and reflectance tympanometry,” Ear and Hearing, 17:361-373), which energy reflectance was measured as a function of frequency and static pressure in the ear canal. In view of the description herein, it should be appreciated that the principles of how an immittance tympanometry device operates in calibration and ear-testing modes differ substantially from the principles of how a reflectance tympanometry device operates.
The above-referenced patents, for example, describe procedures for varying static pressure in the ear canal so as to perform reflectance tympanometry. For example, FIG. 9 of U.S. Pat. No. 5,792,072 shows a system to measure reflectance as a function of static pressure and frequency comprised of a computer, an ear canal estimate storage area, and a tympanometer. The tympanometer is comprised of a probe assembly, a static pressure pump, and a tympanometer processor. However, unlike conventional admittance tympanometers which are known to use various feedback systems to control static pressure, the system measures acoustic reflectance without knowing what the static pressure is in the ear canal.
Published reports of measurements of acoustic reflectance of the ear as a function of static pressure and frequency share the same limitation, e.g., they lack the ability to control or measure static pressure in any fashion; see, for example, Keefe D H and Levi E (1996) (“Maturation of the middle and external ears: Acoustic power-based responses and reflectance tympanometry,” Ear and Hearing, 17:361-373). Further, for example, in Margolis, et al., “Wideband reflectance tympanometry in normal adults,” J. Acoust. Soc. Am. 106:265-280 (1999) and in Margolis, et al., “Wideband reflectance tympanometry in chinchillas and humans,” J. Acoust. Soc. Am. 110: 1453-1464 (2001), researchers measured reflectance by varying static pressure manually. Yet further, acoustic reflectance tympanometry was also performed as described in Keefe and Simmons (2003) (“Energy transmittance predicts conductive hearing loss in older children and adults,” J. Acoust. Soc. Am. 114, 3217-3238) using a device similar to that described in U.S. Pat. No. 5,792,072 (e.g., a computer commands a static pressure pump to an intended static pressure by supplying a DC voltage to a pump controller).
However, if the intended static pressure is not equal to the actual static pressure in the ear canal when reflectance tympanometry is carried out, then measurement errors occur in such tests. Due to the different manner in which the probe fits in the ear canal in different patients, it may never be possible to know the extent of the errors (e.g., the actual static pressure in the ear canal may be very different than actually intended or even unknown). In fact, whenever there is a leaky fit of the probe assembly into the ear canal, the resulting static pressure in the ear canal may remain at atmospheric pressure independent of the intended static pressure in the device and the resulting measurements may be unusable. Such a lack in accuracy of acoustic reflectance measurements, affects the reliability of the use of such acoustic reflectance systems in clinical applications (e.g., in the screening and diagnosis of middle-ear dysfunction and conductive hearing loss in human patients).
A conductive hearing loss is a hearing loss associated with sound conduction through the middle-ear; some types of middle-ear dysfunction are pathologies that include a conductive hearing loss but other types of middle-ear dysfunction do not include a conductive hearing loss. A child with a conductive hearing loss tends to have associated developmental delays in language acquisition and cognitive skills, so that an improved method to screen and diagnose a conductive hearing loss would be important in efforts to ameliorate the hearing loss, minimize developmental delays, and minimize the resulting personal, family and societal costs associated with an extended period of an unrecognized conductive hearing loss. Conventional admittance tympanometry is inefficient at detecting a conductive hearing loss, whereas acoustic reflectance is capable of detecting the presence of a conductive hearing loss.
The above discussion noted that acoustic-reflex testing in the form of a shift in acoustic immittance has been a well known part of an acoustic immittance test battery for decades. Other types of acoustic-reflex testing, which do not rely on acoustic immittance tests as practiced within the scope of ANSI S3.39-1987 and related audiological publications, have recently been developed. These non-immittance types of MEMR tests fall into two classes based on a shift in a response in the presence of a MEMR activator signal: (1) a MEMR test based on a shift in sound pressure, which may be any shift in the magnitude and/or phase response in the frequency domain or in the waveform of a time-domain response, and (2) a MEMR test based on a shift in a wideband acoustic reflectance response (or in related acoustic absorbed power, transmittance or acoustic intensity responses).
Yet further, in many circumstances, existing individual tests of the functional status of the auditory system (e.g., such as a test of acoustic reflectance to assess the peripheral auditory system) provides incomplete and limited information. For example, the use of an admittance tympanogram to assess the status of the external and middle-ear may not provide desired information about middle-ear functioning in newborns because of the mobility of the ear-canal walls, about the likelihood of a conductive hearing loss, or about the presence of middle-ear dysfunction that affects middle-ear functioning at a frequency higher than that used in the admittance tympanogram. Further, for example, although a conventional middle-ear muscle reflex (MEMR) test may provide an indirect measure of cochlear and auditory nerve functioning, such a test provides a high false-positive rate (e.g., some normal-hearing ears lack a MEMR shift), in part, due to limitation of current testing procedures.
Various apparatus have been described which may be used to perform a battery of tests on a patient. For example, U.S. Pat. No. 6,974,421 to Causevic et al. issued 13 Dec. 2005, entitled “Handheld audiometric device and method of testing hearing,” describes an apparatus that may perform a battery of tests, either independently or combined on a patient. Such tests are described as including an otoacoustic measurements utilizing digital signal processing for evoked otoacoustic signal processing, an auditory brain stem response (ABR) test, admittance tympanometry, and otoreflectance. Further, for example, U.S. Pat. No. 5,601,091 to Dolphin issued 11 Feb. 1997, entitled “Audiometric apparatus and association screening method;” as well as U.S. Pat. No. 5,916,174 to Dolphin issued 29 Jun. 1999, entitled “Audiometric apparatus and associated screening method,” describe apparatus that may perform multiple tests including an otoacoustic emission (OAE) test, an ABR test, and even an acoustic reflectivity test. Yet further, for example, International Publication No. WO 03/099121 A2, entitled “Systems and methods for conducting multiple diagnostic hearing tests” describes a system that is capable of performing audiometry tests, an otoacoustic emission test, and acoustic immittance tests including admittance tympanometry and reflex testing based on a shift in acoustic immittance.
However, although apparatus previously described may perform a battery of specifically described tests, such combinations of tests still do not provide sufficient information indicative of various auditory functions and/or the impairment of such auditory function. For example, the performance of such tests may not provide sufficient information for the screening of newborn infants at high frequencies, for use in the diagnosis of auditory neuropathy, or for use in the fitting of a hearing aid. As such, improvement in the types of apparatus that combine a battery of tests is needed.